The invention set forth in this specification pertains to sandblasting with pellets of material or materials capable of sublimation.
"Sandblasting" is considered to be a generic term used to designate any of a series of processes in which pellets or particles are propelled against a surface by entrainment within a stream of pressurized gas so as to effect a change at or on such a surface. Conventional sandblasting is commonly utilized for cleaning various different types of surfaces of various different types of contaminants. On occasion sandblasting is utilized for other purposes such as, for example, to alter a physical condition of a material adjacent to the surface of the material. Thus, the term "sandblasting" is occasionally utilized to designate various peening processes in which particles are propelled against a metal surface in order to develop desired physical properties within such a surface.
It has been recognized that the use of particles which are solid under normal ambient conditions in sandblasting processes is disadvantageous because of problems relating to cleaning up such particles after they have been used in sandblasting and problems relating to the possibility of atmospheric contamination. Such recognition has led to a recognition of the desirability of utilizing solid carbon dioxide or dry ice particles in sandblasting processes.
It has been considered that the use of such dry ice particles in sandblasting would be advantageous because such solid carbon dioxide particles will disappear as a gas after having been used in sandblasting. Obviously there are no cleanup problems attendant to the removal of gaseous carbon dioxide after a surface has been sandblasted with solid carbon dioxide particles. Further, gaseous carbon dioxide only presents a minimal atmospheric pollution problem inasmuch as ambient air contains carbon dioxide and inasmuch as carbon dioxide gas is readily dispersed within ambient air.
Although the theory behind the use of solid carbon dioxide particles for sandblasting is considered to be basically sound a number of problems have been encountered in effectively sandblasting with dry ice pellets. These problems have primarily concerned the uniform entrainment of such particles within a compressed gas stream under such conditions that the particles are uniformly applied by the gas stream through a nozzle against a surface in such a manner as to maximize the effect of the particles at or on the surface. Perhaps this can best be illustrated by referring to certain problems as have been encountered in sandblasting with carbon dioxide pellets.
One of these problems has concerned the fact that the impingement of a dry ice particle against a surface will normally have a somewhat limited effect at or on the surface as a consequence of the limited density of the particle and as a consequence of the usual or normal somewhat rounded edge and corner configurations of such a particle. Further, the results achieved in sandblasting with dry ice pellets or particles have tended to be somewhat non-uniform as a result of such particles or pellets not feeding in a consistent, uniform manner into streams of compressed gas. To a large extent the latter is related to the fact that such pellets have tended to agglomerate or clump together before and as they have been distributed into a stream of compressed gas. As a consequence of this the pellet or particle content of a gas stream used will vary and will not be consistent. This will tend to make it impossible to minimize the time required to treat a surface with such pellets or particles to the maximum desired extent. On occasion this may even render a sandblasting process using dry ice particles inoperative from a practical standpoint. While to a degree such agglomeration of particles or pellets can be regulated by regulating the purity of the pellets or particles used this expedient is not considered to completely remedy the noted problem.